Megasonic cleaning techniques are known in the art and are presently widely applied in the semiconductor industry. The performance of megasonic cleaning depends on several process parameters such as frequency, dissolved gas in the cleaning liquid, applied power, acoustic field geometry, refreshing of the cleaning liquid, wafer positions, and chemical concentrations. Despite the wide application of megasonic cleaning, the importance of each individual process parameter is still not sufficiently clear.
Even the physical mechanisms that are responsible for the removal of particles (such as Schlichting streaming, microstreaming and acoustic cavitation) are still under debate, for at least the reason that basic physical processes and forces involved in the cleaning process are little understood. All these uncertainties make it very difficult to optimize megasonic cleaning tools to the ever more stringent requirements of the semiconductor industry. The process window of physical forces in which structures can be cleaned without creating damage is continuously narrowing. As a result, a precise control over the physical forces during megasonic cleaning is extremely important.
In WIPO Application Pub. No. WO-A-2004071938 (Boyd et al.), a method and apparatus are disclosed involving a megasonic transducer oriented towards a substrate. A dynamically adjustable RF generator is provided that has an output coupled to the transducer, and the RF generator is adjusted to maintain a resonance state of the transducer, thereby obtaining improved cleaning efficiency. While this approach provides an improvement over systems wherein no such control is applied, a further optimization is desirable.
The present invention aims to provide further improvement over the systems described above.